Recent advances in WLED technology (White Light Emitting Diode) have made large arrays of WLEDs cost effective for backlighting applications in LCD screens for notebook computers, personal digital assistants, and flat panel televisions. There are applications now that already require hundreds of WLEDs and it is likely that thousands of WLEDs will be employed in future applications.
These large arrays often involve parallel connections of series strings of WLEDs. For instance a typical array might have twelve parallel strings where each string has ten WLEDs in series for a total number of one hundred twenty WLEDs.
FIG. 1 (Prior Art) is a diagram of an LCD display 1 that employs twelve strings of WLEDs for backlighting. The forward voltage drop across a WLED is typically from two to four volts. The forward current through a WLED can vary over several orders of magnitude but for many WLED arrays used for personal portable electronics a forward current of ten to one hundred milliamperes is common. To a first order the brightness of a WLED is proportional to the current passing through it. In order to provide constant brightness across a large array of WLEDs, the current through each string of WLEDs should be well matched to the other strings in the same array. Typical requirements for current matching are less than two percent over the whole array.
FIG. 2 (Prior Art) shows one way of providing current matching between multiple strings of WLEDs. In LCD display 2, the number of WLED strings is cut down to one and the current is run through the one long string of WLEDs. In theory, this gives the perfect current matching because exactly the same current flows through all the WLEDs (neglecting leakages and other second order effects). However, as array sizes increase and the subsequent number of WLEDs increases into the hundreds, the voltage V+ required to forward bias the LEDs becomes quite high. Providing this high voltage V+ requires specialized circuitry and creates new problems due to leakages. Accordingly, for practical applications, a parallel combination of multiple strings of WLEDs is advantageous. This necessitates that each string has a current control element in order that the current will match from string to string.
FIG. 3 (Prior Art) is a diagram that illustrates an array 3 of four strings 8-11 of WLEDs that are controlled by four corresponding respective current sources 4-7. An integrated circuit solution may include four to eight such current sources on one integrated circuit die. In FIG. 3, the current sources 4-7 are disposed on one integrated circuit die 12. If larger numbers of WLED strings are required, however, then such multiple integrated circuits may be used.
FIG. 4 (Prior Art) illustrates one problem with providing multiple such integrated circuits 13-15 to control a corresponding large number of WLED strings. The problem is that a current reference of one integrated circuit 13 may not match a current reference of another integrated circuit 15 used in the same large array. If there is a mismatch between the current references of the two integrated circuits, then there will also be a mismatch between the current flow through different WLED strings in the strings of WLEDs in the same LCD display. That will cause a brightness mismatch across the LCD display that might be detectable to the human eye and therefore unacceptable to humans who are considering purchasing the display. This is undesirable.
Techniques are known for making on-chip current references more accurate. These techniques usually involve some type of trimming in order provide the tight control necessary for these applications.
FIG. 5 (Prior Art) is a diagram that illustrates one such technique employing resistor trimming. To make all the currents I1-IN identical that flow into integrated circuit 16, the resistors 17-19 are trimmed. In one example, a “thin film resistor” layer is added to the integrated circuit to provide the low temperature coefficient and trimmability required. Any form of trimming, whether it be laser trimming or blowing fuse links on the integrated circuit or some other way trimming, will increase the cost of the integrated circuit.
FIG. 6 (Prior Art) illustrates another technique for providing precisely controlled current sources between multiple integrated circuits. This technique involves requiring each of the integrated circuits to have an associated highly accurate external resistor and an accurate internal voltage source. In the illustration of FIG. 6, integrated circuit 20 has an accurate internal voltage source 21 and an external precision resistor 22, and integrated circuit 23 has an accurate internal voltage source 24 and an external precision resistor 25. Drawbacks to this technique include the added cost of the external resistors, the variation and mismatch of the external resistors, and variations of the internal voltage sources. Providing the internal voltage sources with adequate precision generally requires some form of trimming. Moreover, voltage drops across the common ground line 26 may also lead to more current mismatch between multiple integrated circuits.
FIG. 7 (Prior Art) illustrates a further extrapolation of these potential solutions. The voltage reference is produced by only one circuit 27 and is bussed to all the integrated circuits 28-30. Each integrated circuit 28-30 has its associated accurate external resistor 31-33 as in the circuit of FIG. 6. The circuit of FIG. 7 has problems because voltage drops along the ground line 34 of a large LCD display would effectively change the reference voltage that each integrated circuit “sees” as one moves along the ground line 34 from integrated circuit to integrated circuit.